1. Field of the Invention
The present invention relates generally to a valve timing control apparatus that controls the timing of the intake or exhaust valves of an engine in accordance with the running condition of the engine. More particularly, the present invention pertains to a valve timing control apparatus that controls the valve timing to adjust the valve overlap.
2. Description of the Related Art
A conventional engine generally has intake and exhaust valves, which selectively open and close intake ports and exhaust ports leading into combustion chambers. The timing of the valves is synchronized with the rotational phase of a crankshaft, or the reciprocal movement of pistons. The amount of gas drawn into the combustion chambers (intake amount) and the amount of exhaust gas discharged from the combustion chambers (exhaust amount) are determined by the opening of a throttle valve or the engine speed. As shown in FIG. 9(a) and 9(b), during a cycle in which the crankshaft is rotated for an angle of 720 degrees, there is a range where the intake and exhaust valves of each combustion chamber are opened at the same time. This so-called valve overlap assists in the intake of the air-fuel mixture into the combustion chamber and the exhaustion of the combusted gas from the combustion chamber.
Various control apparatuses that enable alteration of the valve timing to adjust the intake and exhaust amount with a greater degree of freedom have recently become available. Such apparatuses typically include a variable valve timing mechanism for alteration of the valve timing and a computer to control the operation of the timing mechanism. The computer controls the valve timing of either the intake valve or the exhaust valve in accordance with the running condition of the engine to adjust the valve overlap of the intake and exhaust valves. This enables the air-fuel ratio of the air-fuel mixture drawn into the combustion chamber to be adjusted to an appropriate value and thus enables the power of the engine to be controlled.
For instance, when the engine speed is relatively high, the computer relatively prolongs the valve overlap range with the timing mechanism. This improves the engine power by enhancing the charging efficiency of the intake gas in the combustion chambers with the inertia effect of the air flowing in from an air intake passage. When the engine speed is rather low, the computer relatively shortens the valve overlap range with the timing mechanism. This prevents a reversed flow of the exhaust gas discharged from each combustion chamber. Thus, the reduced amount of residual gas in each combustion chamber, or the lowered ratio of internal exhaust gas recirculation (EGR), prevents insufficient combustion of the air-fuel mixture.
An example of such a control apparatus is described in Japanese Unexamined Patent Publication No. 4-228843. The apparatus controls the valve timing of the intake and exhaust valves in accordance with the running condition of the engine. As shown in FIG. 10, the control apparatus includes an electronic control unit (ECU) 91 which controls first, second, third, and fourth electromagnetic valves 96, 97, 98, 99 in response to values detected by engine speed, intake pressure, vehicle velocity, and coolant temperature sensors 92, 93, 94, 95. This enables the ECU 91 to control timing pulley assemblies 102, 103, which alter the rotational phase of the associated cam shafts 100, 101. Consequently, the valve timing of each intake and exhaust valve (not shown) is altered and the valve overlap is adjusted.
The ECU 91 judges how warm the engine is from the coolant temperature. The ECU 91 controls the electromagnetic valves 96, 97, 98, 99 to eliminate the valve overlap when it determines that the coolant temperature is lower than a predetermined reference value. This prevents air-fuel mixture and combusted gas (exhaust gas) from simultaneously entering the engine's combustion chambers (not shown). Thus, the fuel mist suspended in the air-fuel mixture is prevented from adhering to spark plugs and thus does not interfere with ignition of the spark plugs despite the low temperature.
Elimination of valve overlap under low temperature conditions not only contributes to better ignition of the spark plugs, but also suppresses the amount of the so-called port wet. Port wet is a state in which fuel adheres to the walls of intake ports that lead into combustion chambers. In this case, port wet especially refers to the fuel that adheres to the walls of the intake ports due to a reversed flow of air-fuel mixture from the combustion chambers. This prevents the air-fuel ratio of the mixture in the combustion chamber from being changed to an undesirable value and thus enables a reduction of hydrocarbon (HC) in the exhaust gas.
However, in the above prior art control apparatus, the ECU 91 determines the state of valve overlap by judging whether the temperature is low by merely comparing it with a predetermined reference value. Therefore, switching from a non-overlap state to an overlap state may create undesirable engine running conditions. In other words, there is a difference in the amount of port wet when comparing a valve overlap state and a non-overlap state. Additionally, the difference between the port wet amount in both states changes according to the coolant temperature. Therefore, switching the state of the valve overlap by merely judging whether the temperature is lower than a reference value may cause formation of an excessive amount of port wet after the switching. In such cases, the amount of fuel supplied to the combustion chambers after the valve overlap switching may be insufficient and result in an undesirable air-fuel ratio inside the combustion chamber. This leads to a degradation in the engine performance and emissions.